Medium and method for providing a desirable direct overwriting characteristic for optical data recorded at different linear recording velocities

ABSTRACT

It is an object of the present invention to provide a method for recording information in an optical recording medium with a good direct overwriting characteristic. The method for recording information in an optical recording medium according to the present invention forms record marks in a recording layer containing a phase change material by modulating the power of a laser beam between a plurality of power levels including a recording power Pw, an erasing power Pe and ground power Pb. The erasing power Pe is set to a level within a region where a reflection coefficient of a region between neighboring record marks approaches a reflection coefficient of the record mark as the erasing power Pe increases. As a result, a good overwriting characteristic can be obtained when data are recorded at a high linear velocity.

BACKGROUND OF THE INVENTION

The present invention relates to a method for recording data in anoptical recording medium and, particularly, to a method for recordingdata in a rewritable type optical recording medium whose recording layercontains a phase change material. The present invention also relates toan optical recording medium and, particularly, to a rewritable typeoptical recording medium whose recording layer contains a phase changematerial.

DESCRIPTION OF THE PRIOR ART

Optical recording media such as the CD, DVD and the like have beenwidely used as recording media for recording digital data and a datarecording method for recording data by modulating data to be recordedinto lengths of record marks along a track has been widely used. Forexample, in a DVD-RW, which is a kind of optical recording media inwhich data can be rewritten by a user, record marks whose lengthscorrespond to 3T to 11T and 14 T, where T is a clock period, are usedfor recording data.

When such record marks are to be formed, a laser beam is projected onthe optical recording medium along its track, whereby an amorphousregion of a predetermined length to be used as a record mark is formedin a recording layer formed of a phase change material in the opticalrecording medium. Regions other than the amorphous region arecrystallized.

When a record mark is to be formed in the recording layer, a laser beamwhose power has been set at a sufficiently high level, namely, arecording power Pw, is projected onto the recording layer to heat it toa temperature higher than the melting point thereof and the recordinglayer is then quickly cooled by changing the power of the laser beam toa sufficiently low level, namely, a ground power Pb. As a result, thephase of the phase change material is changed from a crystal phase to anamorphous phase, thereby forming a record mark. On the other hand, whena record mark is to be erased, a laser beam whose power has been set toa level equal to or lower than the recording power Pw and equal to orhigher than the ground power Pb, namely, an erasing power Pe, isprojected onto the recording layer to heat it up to a temperature equalto or higher than the crystallization temperature thereof and therecording layer is gradually cooled, whereby the phase of the phasechange material is changed from an amorphous phase to a crystal phaseand the record mark is erased. Therefore, it is possible not only toform a record mark in an unrecorded region of the recording layer butalso to directly overwrite a record mark with a different record mark ina recorded region by modulating the power of the laser beam between aplurality of levels including the recording power Pw, the erasing powerPe and the ground power Pb.

The optimum level of the erasing power Pe mainly depends upon thecomposition of the phase change material contained in the recordinglayer as a primary component and, therefore, the optimum level of theerasing power Pe generally changes depending upon the composition of thephase change material contained in the recording layer. For this reason,when an optical recording medium is designed, ordinarily a mixed signalfor forming a plurality of record marks is recorded onto the same trackof a prototype optical recording medium a plurality of times and theerasing power Pe at which a reflection coefficient of a blank regionbetween record marks neighboring in the direction of a track becomessaturated is determined to be the optimum level of the erasing power Pe.

SUMMARY OF THE INVENTION

However, the inventors of the present invention vigorously pursued astudy and, as a result, made the discovery that in the case where theerasing power Pe determined by this method is used, jitter ofoverwritten data may not be good depending upon the linear velocity ofdata recording (recording linear velocity). Therefore, a good directoverwriting characteristic can be obtained at a certain recording linearvelocity while a good direct overwriting characteristic cannot beobtained at another recording linear velocity.

It is therefore an object of the present invention to provide a methodfor recording data in an optical recording medium with a good directoverwriting characteristic and an optical recording medium in which datacan be recorded with a good direct overwriting characteristic.

It is another object of the present invention to provide a method forrecording data in an optical recording medium with a good directoverwriting characteristic irrespective of recording linear velocity andan optical recording medium in which data can be recorded with a gooddirect overwriting characteristic irrespective of recording linearvelocity.

In a study carried out by the inventors of the present invention, it wasfound that even if the recording layers have the exactly the samecomposition, the relationship between the erasing power Pe and thereflection coefficient of a blank region between record marksneighboring in the direction of a track greatly changes depending uponthe recording linear velocity.

FIG. 1 is a graph schematically showing the relationship between theerasing power Pe and reflection coefficient of a region between recordmarks neighboring in the direction of a track at a low recording linearvelocity and shows the reflection coefficient of the region betweenrecord marks neighboring in the direction of a track after a mixedsignal for forming a plurality of record marks was recorded on the sametrack of the optical recording medium a plurality of times under variouslevels of the erasing power Pe.

The area indicated by the symbol {circle around (1)} in FIG. 1 is anarea that remained mostly in an amorphous phase since the erasing powerPe of the laser beam was too weak and the temperature of the recordinglayer did not reach the crystallization temperature thereof. Therefore,if the actual erasing power Pe of the laser beam is set to a levelwithin this area, record marks cannot be completely erased. The areaindicated by a symbol {circle around (2)} in FIG. 1 is an area in whichsubstantially all of the amorphous region was crystallized since thetemperature of the recording layer reached the crystallizationtemperature thereof. Therefore, if the actual erasing power Pe of thelaser beam is set to a level within this area, record marks can beerased while maintaining their solid phase. The area indicated by thesymbol {circle around (4)} in FIG. 1 is an area in which substantiallyall of the amorphous region was crystallized since the temperature ofthe recording layer reached the melting point thereof. Therefore, if theactual erasing power Pe of the laser beam is set to a level within thisarea, record marks can be melted and erased.

The area indicated by the symbol {circle around (3)} in FIG. 1 is anarea located between the area {circle around (2)} and the area {circlearound (4)} and in which substantially all of the amorphous region wascrystallized since the temperature of the recording layer partly reachedthe melting point thereof. Therefore, if the actual erasing power Pe ofthe laser beam is set to a level within this area, record marks can beerased in a mode between the solid-state erasure and the melt-quencherasure.

In the case where the direct overwriting is conducted at the linearvelocity (low linear velocity) at which the relationship between theerasing power Pe and the reflection coefficient of a region between therecord marks neighboring in the direction of a track is as shown in FIG.1, jitter property can be improved by setting the actual erasing powerPe to a level within the areas {circle around (2)} to {circle around(4)}.

FIG. 2 is a graph schematically showing the relationship between theerasing power Pe and reflection coefficient of a region between recordmarks neighboring in the direction of a track at a high recording linearvelocity and, similarly to FIG. 1, shows the reflection coefficient of aregion between record marks neighboring in the direction of a trackafter a mixed signal for forming a plurality of record marks wasrecorded on the same track of the optical recording medium a pluralityof times under various levels of the erasing power Pe.

As can be seen from FIG. 2, when the recording linear velocity was high,the reflection coefficient of a region between record marks neighboringin the direction of a track decreased in the area {circle around (3)}.In the area {circle around (3)}, an area where the reflectioncoefficient of the region between record marks neighboring in thedirection of a track decreased as the erasing power Pe increased isreferred to as area “{circle around (3)}-A” and an area where thereflection coefficient of the region between record marks neighboring inthe direction of a track increased as the erasing power Pe increased isreferred to as area “{circle around (3)}-B.” The inventors of thepresent invention found that jitter property can be most improved bysetting the actual erasing power Pe to a level within the area “{circlearound (3)}-A.”

Although the reason why jitter property can be most improved by settingthe actual erasing power Pe within the area “{circle around (3)}-A” isnot clear, it is reasonable to conclude that since the recording layerbegins to be melted by a laser beam whose erasing power Pe is in thearea “{circle around (3)}-A,” record marks not sufficiently erased bythe solid-state erasure at a high linear velocity in the area {circlearound (2)} can be desirably erased.

The present invention is directed to solving the above describedproblems based on this finding. A method for recording data in anoptical recording medium according to one aspect of the presentinvention is adapted for recording data in an optical recording mediumincluding a recording layer containing a phase change material byprojecting a laser beam thereonto and comprises the steps of projectingthe laser beam onto the recording layer so that the portion of therecording layer onto which the laser beam impinges is continuously movedand modulating the power of the laser beam between a plurality of powerlevels including at least an erasing power, thereby recording aplurality of record marks in the recording layer, the level of theerasing power being set to a level within an area where a reflectioncoefficient of a region between record marks neighboring in thedirection of a track approaches a reflection coefficient of the recordmark as the erasing power increases.

According to this aspect of the present invention, a good overwritingcharacteristic can be obtained when data are recorded at a high linearvelocity, thereby improving jitter property of the thus formed recordmark.

In a preferred aspect of the present invention, the plurality of powerlevels includes a recording power whose level is higher than that of theerasing power and a ground power whose level is lower than that of theerasing power.

In a preferred aspect of the present invention, the optical recordingmedium is constituted as a “high-to-low” type and the level of theerasing power is set to a level within which the reflection coefficientof a region between record marks neighboring in the direction of a trackdecreases as the erasing power increases.

A method for recording data in an optical recording medium according toanother aspect of the present invention is adapted for recording data inan optical recording medium including a recording layer containing aphase change material by projecting a laser beam thereonto and comprisesthe steps of projecting the laser beam onto the recording layer so thatthe portion of the recording layer onto which the laser beam impinges iscontinuously moved and modulating the power of the laser beam between aplurality of power levels including at least an erasing power, therebyrecording a plurality of record marks in the recording layer, the levelof the erasing power being set to higher than a level within an areawhere the reflection coefficient of the region between the record marksneighboring in the direction of a track is not substantially changedeven if the erasing power increases and lower than a level within anarea where the reflection coefficient of the region between the recordmarks neighboring in the direction of a track moves away from that ofthe record mark as the erasing power increases.

A method for recording data in an optical recording medium according toa further aspect of the present invention is adapted for recording datain an optical recording medium by modulating the power of a laser beambetween a plurality of power levels including at least erasing power andrecording record marks in a recording layer containing a phase changematerial wherein the erasing power is set to a first level when data areto be recorded at a first linear velocity and the erasing power is setto a second level different from the first level when data are to berecorded at a second linear velocity higher than the first linearvelocity. According to this aspect of the present invention,multi-velocity recording of data can be accomplished for an opticalrecording medium.

In a preferred aspect of the present invention, the first linearvelocity is set so that the reflection coefficient of the region betweenrecording marks neighboring in the direction of a track becomessubstantially saturated when the erasing power is set to be equal to orhigher than a predetermined level and the second linear velocity is setso that the reflection coefficient of the region between the recordmarks neighboring in the direction of a track approaches the reflectioncoefficient of the record mark as the erasing power increases when dataare recorded at the second linear velocity.

In a further preferred aspect of the present invention, the first levelis set so that the reflection coefficient of the region between therecord marks neighboring in the direction of a track becomes equal to afirst value and the second level is set so that the reflectioncoefficient of the region between the record marks neighboring in thedirection of a track becomes equal to a second value closer to thereflection coefficient of the record mark than the first value is.

An optical recording medium according to one aspect of the presentinvention is provided with a recording layer which contains a phasechange material and in which record marks are to be formed byirradiation with a laser beam and with predetermined informationnecessary for setting an erasing power of the laser beam to a levelwithin an area where a reflection coefficient of a region between recordmarks neighboring in the direction of a track approaches the reflectioncoefficient of the record mark as the erasing power increases.

An optical recording medium according to another aspect of the presentinvention is provided with a recording layer which contains a phasechange material and in which record marks are to be formed byirradiation with a laser beam, and with first predetermined informationnecessary for setting a level of an erasing power corresponding to afirst recording linear velocity and second predetermined informationnecessary for setting a level of the erasing power corresponding to asecond recording linear velocity higher than the first recording linearvelocity.

In a preferred aspect of the present invention, when data are recordedat the first recording linear velocity, the reflection coefficient ofthe region between the record marks neighboring in the direction of atrack becomes equal to a first value and when data are recorded at thesecond recording linear velocity, the reflection coefficient of theregion between record marks neighboring in the direction of a trackbecomes equal to a second value that is closer to the reflectioncoefficient of the record mark than the first value is.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph schematically showing the relationship between erasingpower Pe and reflection coefficient of a region between record marksneighboring in the direction of a track at a low recording linearvelocity.

FIG. 2 is a graph schematically showing the relationship between erasingpower Pe and reflection coefficient of a region between record marksneighboring in the direction of a track at a high recording linearvelocity.

FIG. 3 is a cross-sectional view schematically showing the structure ofan optical recording medium 10 which is a preferred embodiment of thepresent invention.

FIG. 4 is a diagram showing one example of a pulse pattern of a laserbeam for forming record marks whose length corresponds to 3T.

FIG. 5 is a diagram showing one example of information for settingrecording conditions.

FIG. 6 is a graph schematically showing how reflection coefficient of aregion between record marks neighboring in the direction of a trackvaries with magnitude of erasing power Pe at high linear velocity in alow-to-high type optical recording medium.

FIG. 7 is a graph showing how the highest reflection coefficient varieswith magnitude of erasing power Pe at recording linear velocity of 3.5m/sec.

FIG. 8 is a graph showing how jitter varies with magnitude of erasingpower Pe at recording linear velocity of 3.5 m/sec.

FIG. 9 is a graph showing how the highest reflection coefficient varieswith magnitude of erasing power Pe at recording linear velocity of 7.0m/sec.

FIG. 10 is a graph showing how jitter varies with magnitude of erasingpower Pe at recording linear velocity of 7.0 m/sec.

The above and other objects and features of the present invention willbecome apparent from the following description made with reference tothe accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will bedescribed in detail with reference to accompanying drawings.

FIG. 3 is a cross-sectional view schematically showing the structure ofan optical recording medium 10 that is a preferred embodiment of thepresent invention, the structure being shown for the case where thepresent invention is applied to a DVD type optical recording medium. Asexplained later, the optical recording medium to which a method forrecording data according to the present invention can be applied is notlimited to the DVD type optical recording medium shown in FIG. 3 but maybe an optical recording media having another structure.

As shown in FIG. 3, an optical recording medium 10 according to thisembodiment is constituted by a translucent substrate 11, a firstdielectric layer 12 provided on the translucent substrate 11, arecording layer 13 provided on the first dielectric layer 12, a seconddielectric layer 14 provided on the recording layer 13, a reflectivelayer 15 provided on the second dielectric layer 14, a protective layer16 provided on the reflective layer 15 and a dummy substrate 17 providedon the protective layer 16 via an adhesive layer (not shown) and thecentral portion of the optical recording medium 10 is formed with a hole18. In the optical recording medium 10 having such configuration, dataare recorded or reproduced by projecting a laser beam from the side ofthe translucent substrate 11.

The translucent substrate 11 constitutes an incident surface of thelaser beam and has a thickness of about 0.6 mm. The surface of thetranslucent substrate 11 on the opposite side from the incident surfaceis formed with pregrooves (not shown). The material for forming thetranslucent substrate 11 is not particularly limited and polycarbonatecan be used for forming the translucent substrate 11.

The first dielectric layer 12 mainly serves as a protective layer forthe recording layer 13 formed thereabove and preferably has a thicknessof 40 to 250 nm. The material for forming the first dielectric layer 12is not particularly limited and the first dielectric layer 12 can beformed of an oxide, nitride, sulfide or carbide of silicon (Si),aluminum (Al), tantalum (Ta), zinc (Zn) or the like, such as SiO₂,Si₃O₄, Al₂O₃, AlN, TaO, ZnS.

The recording layer 13 is formed of a phase change material and data arerecorded in the recording layer 13 utilizing difference in reflectioncoefficients between crystalline structure and amorphous structure. Aregion of the recording layer 13 can be changed from a crystalline stateto an amorphous state by modulating the power of the laser beamprojected from the side of the translucent substrate 11 in accordancewith a pulse wave having an amplitude between a recording power Pw and aground power Pb, heating the region of the recording layer 13 to atemperature equal to or higher than the melting point thereof andquickly cooling the region. As a result, the region of the recordinglayer 13 melted by the laser beam having the recording power Pwischanged to an amorphous state, thereby forming a record mark. On theother hand, a region of the recording layer 13 can be changed from anamorphous state to a crystalline state by projecting the laser beamhaving an erasing power Pe from the side of the translucent substrate 11and heating the region of the recording layer 13 to a temperature equalto or higher than the crystallization temperature thereof. The regionheated to a temperature equal to or higher than the crystallizationtemperature by the laser 4 beam having the erasing power Pe is graduallycooled and the region is changed to a crystalline state.

The recording power Pw, the erasing power Pe and the ground power Pb areset so that the erasing power Pe is lower than the recording power Pwand equal to or higher than the ground power Pb.

Therefore, it is possible not only to form a record mark at anunrecorded region of the recording layer 13 but also to directlyoverwrite a record mark with a different record mark in a recordedregion by modulating the power of the laser beam between a plurality oflevels including the recording power Pw, the erasing power Pe and theground power Pb.

The material for forming the recording layer 13 is not particularlylimited but an SbTe system material is preferably used for forming therecording layer 13. As the SbTe system material, SbTe may be used aloneor InSbTeGe, AgInSbTe, AgInSbTeGe containing indium (In), tellurium(Te), germanium (Ge) or the like as an additive may be used. Therecording layer 13 preferably has a thickness of 10 to 30 nm.

Similarly to the first dielectric layer 12, the second dielectric layer14 mainly serves as a protective layer for the recording layer 13 andthe thickness of the second dielectric layer 14 is preferably 5 to 200nm, more preferably 10 to 30 nm. The material for forming the seconddielectric layer 14 is not particularly limited and the seconddielectric layer 12 can be formed of an oxide, nitride, sulfide orcarbide of silicon (Si), aluminum (Al), tantalum (Ta), zinc (Zn) or thelike, such as SiO₂, Si₃O₄, Al₂O₃, AlN, TaO, ZnS.

The reflective layer 15 serves to reflect the laser beam projected fromthe side of the translucent substrate 11 and to emit it through thetranslucent substrate 11, and preferably has a thickness of 20 to 400nm. The material for forming the reflective layer 15 is not particularlylimited but the reflective layer 15 may be preferably formed of alloycontaining silver (Ag) or aluminum (Al) and be formed of gold (Au),platinum (Pt) or the like.

The first dielectric layer 12, the recording layer 13, the seconddielectric layer 14 and the reflective layer 15 can be formed bysputtering on the surface of the translucent substrate 11 on which thepregrooves (not shown) are formed.

The protective layer 16 serves to physically and chemically protect thereflective layer 15 and preferably has a thickness of 5 to 50 μm. Thematerial for forming the protective layer 16 is not particularly limitedbut acrylic resin is preferably used for forming the protective layer16.

The dummy substrate 17 serves to ensure the mechanical strength requiredby the optical recording medium 10 and helps to ensure the thickness(about 1.2 mm) required by the optical recording medium 10. It has athickness of about 0.6 mm. The material for forming the protective layer17 is not particularly limited but polycarbonate can be used for formingthe dummy substrate 17. The dummy substrate 17 is adhered onto thetranslucent substrate 11 formed with the first dielectric layer 12, therecording layer 13, the second dielectric layer 14, the reflective layer15 and the protective layer 16 thereon via an adhesive layer (notshown), thereby forming the optical recording medium 10. If thereflective layer 15 can be protected by the adhesive layer, theprotective layer 16 can be omitted. The adhesive agent used for formingthe adhesive layer is not particularly limited but acrylic resin curableby radical polymerization or cation polymerization in accordance withthe object of the adhesive layer and/or the method of fabricating theoptical recording medium 10 may be used for forming the adhesive layer.

The optical recording medium 10 shown in FIG. 3 is one example of theoptical recording media of the present invention and of an opticalrecording medium to which the recording data methods of the presentinvention can be applied; the optical recording medium according to thepresent invention and the optical recording medium to which therecording data methods of the present invention can be applied are notlimited to the optical recording medium 10 shown in FIG. 3 and havingthe above described structure and may be optical recording media havinganother structure.

For example, the present invention can be applied to a DVD type opticalrecording medium fabricated by bonding two translucent substrates 11,each being formed thereon with the first dielectric layer 12, therecording layer 13, the second dielectric layer 14, the reflective layer15 and the protective layer 16. Further, the present invention can bealso applied to a CD type optical recording medium having a translucentsubstrate having a thickness of about 1.1 mm and no dummy substrate.Moreover, the present invention can be applied to a next-generation typeoptical recording medium fabricated by employing a dummy substratehaving a thickness of about 1.1 mm and formed with pregrooves thereonand providing a very thin light transmission layer having a thickness of10 to 300 nm instead of the translucent substrate 11.

An overview of a method for recording data in the thus constitutedoptical recording medium 10 will now be set out.

Although the method is not particularly limited, an 8, 16 modulationmethod can be used for recording data in the optical recording medium 10according to this embodiment. In the 8, 16 modulation method, data to berecorded are modulated to record marks having lengths corresponding to3T to 11T and 14T, where T is one clock period. The modulation method isnot limited to the 8, 16 modulation method and data can also be recordedby other modulation methods such as the (1, 7) RLL modulation method.

Next, one example of a pulse pattern of a laser beam used in the 8, 16modulation method will be described.

FIG. 4 is a diagram showing a pulse pattern of a laser beam used forforming a record mark whose length corresponds to 3T.

As shown in FIG. 4, in the case of forming a record mark whose lengthcorresponds to 3T, the number of pulses of the laser beam is set to “1”.The number of pulses of the laser beam is defined by how many times thepower of the laser beam is increased to Pw. More specifically, assumingthat the time when the laser beam is located at the start point of arecord mark is ts and the time when the laser beam is located at theterminal end point of the record mark is te, the power of the laser beamis once increased to Pw and then lowered to Pb. The power of the laserbeam is set to be Pe before ts and the increase of the power of thelaser beam is started at ts. The power of the laser beam at te is set toPe or Pb.

In the period of Tpulse, high energy is imparted to the recording layer13 of the optical recording medium 10 and the temperature of therecording layer 13 exceeds the melting point thereof, and in the periodTcl, the recording layer 13 of the optical recording medium 10 isquickly cooled. As a result, a record mark whose length corresponds to3T is formed in the recording layer 13 of the optical recording medium10.

In the case of forming record marks having different lengths, similarlyto the case of forming a record mark whose length corresponds to 3T, thepower of the laser beam for recording data is modulated to Pw, Pe or Pband a record mark having a desired length is formed by (n−2) number ofpulses, where n is a multiple of T and can assume one of 3 to 11 and 14.

Next, the method of setting the erasing power Pe in this embodiment willbe described.

As described above, the erasing power Pe is generally set so that,irrespective of the recording linear velocity, the level of the erasingpower Pe at which a reflection coefficient of a region (blank region)between record marks neighboring in the direction of a track becomessaturated is optimum. However, in the case of overwriting data using apulse pattern of the laser beam designed using the erasing power Pe setin this manner, jitter of overwritten data may become worse dependingupon the recording linear velocity. Therefore, in this embodiment, theerasing power Pe is set by a different method from that in which theerasing power Pe is set based on the saturation of the reflectioncoefficient so that jitter of overwritten data is smallest and theerasing power Pe is set in the following manner.

The erasing power Pe is first set at a predetermined level and a mixedsignal for forming a plurality of record marks is recorded on the sametrack a plurality of times. It is preferable to set the number ofoverwirtings in accordance with Part 1 (PHYSICAL SPECIFICATIONS) of DVDSpecifications for Re-recordable Disk and record the mixed signal on thesame track ten times.

The reflection coefficients of the track are then measured and thehighest reflection coefficient among the thus measured reflectioncoefficients is defined as the reflection coefficient corresponding tothe erasing power Pe. In this case, since the record marks are in theamorphous state and have low reflection coefficients, the highestreflection coefficient must be obtained from a region (blank region)between record marks neighboring in the direction of a track.

This operation is conducted at a fixed recording linear velocity byvarying the erasing power Pe and it is judged whether the relationshipbetween the erasing power Pe and the highest reflection coefficientcoincides with that shown in FIG. 1 or that shown in FIG. 2. If therelationship between the erasing power Pe and the highest reflectioncoefficient coincides with that shown in FIG. 1, in which the reflectioncoefficient hardly changes over the area {circle around (2)} whererecord marks are erased by the solid state erasure to the area {circlearound (4)} where record marks are erased by the melt-quench erasure andthe reflection coefficient is substantially saturated in the area{circle around (2)}, the level of the erasing power Pe may be set to alevel within the areas {circle around (2)} to {circle around (4)}. Onthe other hand, if the relationship between the erasing power Pe and thehighest reflection coefficient coincides with that shown in FIG. 2, inwhich the reflection coefficient decreases substantially in the area{circle around (3)} between the area {circle around (2)} where recordmarks are erased by the solid state erasure and the area {circle around(4)} where record marks are erased by the melt-quench erasure and thereflection coefficient is substantially saturated in the area {circlearound (2)}, the level of the erasing power Pe may be set to a levelwithin the area {circle around (3)}-A. However, in the case where therelationship between the erasing power Pe and the highest reflectioncoefficient coincides with that shown in FIG. 1, since it is impossibleto distinguish the areas {circle around (2)} to {circle around (4)}based on the value of the highest reflection coefficient, it ispreferable to set the level of the erasing power Pe to slightly higherthan the level at which the reflection coefficient is saturated,normally, 1.2 to 1.85 times the level at which the reflectioncoefficient is saturated.

Therefore, when the result of measuring the relationship between theerasing power Pe and the highest reflection coefficient at variousrecording linear velocities is that the relationship between the erasingpower Pe and the highest reflection coefficient coincides with thatshown in FIG. 1 at a first recording velocity and the relationshipbetween the erasing power Pe and the highest reflection coefficientcoincides with that shown in FIG. 2 at a second recording velocity,jitter property can be improved at each of the first recording linearvelocity and the second recording linear velocity by setting the levelof the erasing power Pe to the level of a first erasing power in theareas {circle around (2)} to {circle around (4)} in the case of usingthe optical recording medium 10 at the first recording linear velocityand setting the level of the erasing power Pe to the level of a seconderasing power in the area {circle around (3)}-A. Therefore, it ispossible to provide an optical recording medium adapted for multiplevelocities.

It is preferable to store the thus set level of the erasing power Petogether with the corresponding recording linear velocity in the opticalrecording medium 10 as “predetermined information for setting recordingconditions”. If such predetermined information for setting recordingconditions is stored in the optical recording medium 10, thepredetermined information for setting recording conditions can be readout by a drive when a user actually records data and, therefore, it ispossible to adjust the erasing power Pe based on the thus readpredetermined information for setting recording conditions. For example,as shown in FIG. 5, if the predetermined information for settingrecording conditions includes first predetermined informationcorresponding to the first recording linear velocity and secondpredetermined information corresponding to the second recording linearvelocity, when the user issues an instruction to record data at thefirst recording linear velocity, the level of the erasing power Pe inthe drive is set to the level of the first erasing power based on thefirst predetermined information, and when the user issues andinstruction to record data at the second recording linear velocity, thelevel of the erasing power Pe in the drive is set to the level of thesecond erasing power based on the second predetermined information.

It is preferable for the predetermined information for setting recordingconditions to include, in addition to the level of the erasing power Peand the corresponding recording linear velocity, information necessaryfor identifying various conditions such as the pulse pattern requiredfor recording data in the optical recording medium 10. The predeterminedinformation for setting recording conditions may be recorded as wobblesor prepits and may be recorded in the recording layer 13. Thepredetermined information for setting recording conditions is notlimited to information directly indicating respective conditionsnecessary for recording data but may be information which can indirectlyidentify recording conditions by specifying one of the conditions storedin the drive in advance based thereon.

As described above, according to this embodiment, in the case ofrecording data at a linear velocity at which an area where thereflection coefficient of a region between record marks neighboring inthe direction of a track appears between the region where the recordmark is erased by the solid state erasure and the area where the recordmark is erased by the melt-quench erasure, since the erasing power Pe isset to a level within an area where the reflection coefficient of aregion between record marks neighboring in the direction of a trackdecreases as the erasing power Pe increases and the pulse pattern of thelaser beam is designed based on the thus set erasing power Pe, a gooddirect overwriting characteristic can be obtained.

The present invention has thus been shown and described with referenceto specific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the describedarrangements but changes and modifications may be made without departingfrom the scope of the appended claims.

For example, in the above described embodiment, although explanation wasmade as to the case where the present invention is applied to aso-called “high-to-low” type optical recording medium whose reflectioncoefficient is lowered by the formation of a record mark (amorphousrecord mark), the present invention can be applied to a so-calledlow-to-high type optical recording medium in which the reflectioncoefficient of a recording layer is increased by the formation of arecord mark. In this case, as shown in FIG. 6, the erasing power Pe isset to a level within the area {circle around (3)}-A. At any rate, inboth the case of applying the present invention to a “high-to-low” typeoptical recording medium and the case of applying the present inventionto a “low-to-high” type optical recording medium, it is sufficient toset the erasing power to a level within an area where the reflectioncoefficient of a region between record marks neighboring in thedirection of a track approaches the reflection coefficient of a recordmark as the erasing power is increased.

As described above, according to the present invention, a good directoverwriting characteristic can be obtained.

A good overwrite shelf characteristic, namely, a good rewritablecharacteristic of a data recorded portion after long-term storage, isrequired in a rewritable phase change type optical recording medium.According to the present invention, a good overwrite shelfcharacteristic can be ensured even in the case of conducting directoverwriting at a high linear velocity since the erasing power is raisedto higher than usual by setting it to a level within an area where thereflection coefficient of a region between record marks neighboring inthe direction of a track approaches the reflection coefficient of arecord mark as the erasing power increases.

WORKING EXAMPLES Working Example 1

An optical recording medium sample having the structure shown in FIG. 2was fabricated so as to include a translucent substrate 11 formed ofpolycarbonate and having a thickness of about 0.6 mm, a first dielectriclayer 12 formed of a mixture of ZnS and SiO₂ and having a thickness ofabout 80 nm, a recording layer 13 formed of AgInSbTeGe and having athickness of about 15 nm, a second dielectric layer 14 formed of amixture of ZnS and SiO₂ and having a thickness of about 20 nm, areflective layer 15 formed of alloy containing aluminum as a primarycomponent and having a thickness of about 120 nm, a protective layer 16formed of ultraviolet ray curable resin and having a thickness of about5 μm and a dummy substrate 17 formed of polycarbonate and having athickness of about 0.6 mm. The optical recording medium sample wasfabricated by using a sputtering process to sequentially form the firstdielectric layer 12, the recording layer 13, the second dielectric layer14 and the reflective layer 15 on the surface of the translucentsubstrate 11 formed with pregrooves, using a spin coating method to formthe protective layer 16 on the reflective layer 15, and bonding thedummy substrate 17 on the translucent substrate 11 formed with the firstdielectric layer 12, the recording layer 13, the second dielectric layer14, the reflective layer 15 and the protective layer 16.

A mixed signal for forming a plurality of record marks was recorded in apredetermined track of the thus fabricated optical recording mediumsample by fixing the erasing power Pe of the laser beam at apredetermined level in accordance with the conditions shown in Table 1.As shown in Table 1, the recording linear velocity was 3.5 m/sec (DVDuniform velocity). TABLE 1 Clock frequency 26.16 MHz Clock period (1T)38.2 nsec Linear velocity 3.5 m/sec Modulation method 8, 16 modulationData transfer rate 11.08 Mbps (considering efficiency) Channel bitlength 0.133 μm Numerical aperture (NA) 0.6 Wavelength of a laser beam660 mm

Then, the mixed signal for forming a plurality of record marks wasrecorded on the same track nine times in accordance with the conditionsshown in Table 1 and thereafter, clock jitter of the recorded mixedsignal was measured. The fluctuation σ of a reproduced signal wasmeasured using a time interval analyzer and the clock jitter wascalculated as σ/Tw, where Tw was one clock period.

The recording of the mixed signal and the measurement of jitter wererepeated using various pulse patterns of the laser beam and the pulsepattern at which jitter was lowest was identified.

Reflection coefficients of the track in which the mixed signal had beenrecorded by the pulse pattern at which jitter was lowest were measuredand the highest reflection coefficient was defined as a reflectioncoefficient corresponding to the erasing power Pe. The highestreflection coefficient was measured in a region (blank region) betweenneighboring record marks.

This operation was repeated at the recording linear velocity of 3.5m/sec using various erasing powers Pe and the highest reflectioncoefficient corresponding to each of the erasing powers Pe was measured.

FIG. 7 is a graph showing how the highest reflection coefficient variedwith magnitude of the erasing power Pe at the recording linear velocityof 3.5 m/sec. As shown in FIG. 7, when the recording linear velocity was3.5 m/sec, the highest reflection coefficient was substantiallysaturated in the area where the erasing power Pe was equal to or higherthan about 4.5 mW and it was found that the relationship between theerasing power Pe and the highest reflection coefficient was similar tothat shown in FIG. 1.

FIG. 8 is a graph showing how jitter varied with magnitude of theerasing power Pe at the recording linear velocity of 3.5 m/sec. As shownin FIG. 8, when the recording linear velocity was 3.5 m/sec, it wasfound that the jitter was lowest when the erasing power Pe was about 7mW (about 1.56 times the level at which the highest reflectioncoefficient saturated).

Thus, it was confirmed that when data were recorded (overwritten) in theoptical recording medium at the recording linear velocity of 3.5 m/sec,a good direct overwriting characteristic was obtained by setting theerasing power Pe slightly higher than the level at which the highestreflection coefficient saturated.

Working Example 2

The highest reflection coefficient was measured using the opticalrecording medium sample used in Working Example 1 in the manner ofWorking Example 1, except that the recording conditions shown in Table 2were employed. As shown in Table 2, the recording linear velocity was7.0 m/sec (DVD double velocity). TABLE 2 Clock frequency 52.32 MHz Clockperiod (1T) 19.1 nsec Linear velocity 7.0 m/sec Modulation method 8, 16modulation Data transfer rate 22.16 Mbps (considering efficiency)Channel bit length 0.133 μm Numerical aperture (NA) 0.6 Wavelength of alaser 660 mm beam

FIG. 9 is a graph showing how the highest reflection coefficient variedwith magnitude of the erasing power Pe at the recording linear velocityof 7.0 m/sec. As shown in FIG. 9, when the recording linear velocity was7.0 m/sec, the highest reflection coefficient did not changesubstantially in the area where the erasing power Pe was about 4 mW toabout 7 mW but the highest reflection coefficient decreased as theerasing power Pe increased in the area where the erasing power Pe wasabout 7 mW to about 11 mW, while the reflection coefficient increased asthe erasing power Pe increased in the area where the erasing power Pewas equal to or higher than about 11 mW Therefore, it was found that therelationship between the erasing power Pe and the highest reflectioncoefficient was similar to that shown in FIG. 2.

FIG. 10 is a graph showing how jitter varied with magnitude of theerasing power Pe at the recording linear velocity of 7.0 m/sec. As shownin FIG. 10, when the recording linear velocity was 7.0 m/sec, the jitterassumed a low value at two areas. Further, the jitter in the area wherethe erasing power Pe was higher, namely, about 7.5 to 9.5 mW, was lowerthan that in the area where the erasing power Pe was lower, namely,about 5.5 to 6.0 mW, and the jitter was lowest when the erasing power Pewas about 8.5 mW.

It can be seen from FIG. 10 that the erasing power Pe of 8.5 mW at whichthe jitter was lowest is included in the area {circle around (3)}-Awhere the highest reflection coefficient decreased as the erasing powerPe increased.

Thus, it was confirmed that when data were recorded (overwritten) in theoptical recording medium at the recording linear velocity of 7.0 m/sec,a good direct overwriting characteristic was obtained by setting theerasing power Pe to a level within the area where the highest reflectioncoefficient decreased as the erasing power Pe increased.

1. A method for recording data in an optical recording medium includinga recording layer containing a phase change material by projecting alaser beam thereonto, the method comprising steps of projecting thelaser beam onto the recording layer so that the portion of the recordinglayer onto which the laser beam impinges is continuously moved andmodulating the power of the laser beam between a plurality of powerlevels including at least an erasing power, thereby recording aplurality of record marks in the recording layer, the level of theerasing power being set to a level within an area where a reflectioncoefficient of a region between neighboring record marks approaches areflection coefficient of the record mark as the erasing powerincreases.
 2. A method for recording data in an optical recording mediumin accordance with claim 1 wherein the plurality of power levelsincludes a recording power whose level is higher than that of theerasing power and a ground power whose level is lower than that of theerasing power.
 3. A method for recording data in an optical recordingmedium in accordance with claim 1 wherein the optical recording mediumis constituted as a “high-to-low type and the level of the erasing poweris set to a level within which the reflection coefficient of a regionbetween neighboring record marks decreases as the erasing powerincreases.
 4. A method for recording data in an optical recording mediumin accordance with claim 2 wherein the optical recording medium isconstituted as a “high-to-low type and the level of the erasing power isset to a level within which the reflection coefficient of a regionbetween neighboring record marks decreases as the erasing powerincreases.
 5. A method for recording data in an optical recording mediumincluding a recording layer containing a phase change material byprojecting a laser beam thereonto, the method comprising steps ofprojecting the laser beam onto the recording layer so that the portionof the recording layer onto which the laser beam impinges iscontinuously moved and modulating the power of the laser beam between aplurality of power levels including at least an erasing power, therebyrecording a plurality of record marks in the recording layer, the levelof the erasing power being set to higher than a level within an areawhere the reflection coefficient of the region between the neighboringrecord marks is not substantially changed even if the erasing powerincreases and lower than a level within an area where the reflectioncoefficient of the region between the neighboring record marks movesaway from that of the record mark as the erasing power increases.
 6. Amethod for recording data in an optical recording medium in accordancewith claim 5 wherein the optical recording medium is constituted as a“high-to-low type and the level of the erasing power is set to a levellower than a level within an area where the reflection coefficient ofthe region between the neighboring record marks increases as the erasingpower increases.
 7. A method for recording information in an opticalrecording medium for forming record marks in a recording layercontaining a phase change material by modulating the power of a laserbeam between a plurality of power levels including at least erasingpower, the method comprising a step of setting the level of the erasingpower to a level within a region where the reflection coefficient of theregion between the neighboring record marks approaches the reflectioncoefficient of the record mark as the erasing power increases.
 8. Amethod for recording information in an optical recording medium inaccordance with claim 7 wherein the plurality of power levels includes arecording power whose level is higher than that of the erasing power anda ground power whose level is lower than that of the erasing power.
 9. Amethod for recording information in an optical recording medium inaccordance with claim 7 wherein the optical recording medium isconstituted as a “high-to-low type and the level of the erasing power isset to a level within which the reflection coefficient of a regionbetween neighboring record marks decreases as the erasing powerincreases.
 10. A method for recording information in an opticalrecording medium in accordance with claim 8 wherein the opticalrecording medium is constituted as a “high-to-low type and the level ofthe erasing power is set to a level within which the reflectioncoefficient of a region between neighboring record marks decreases asthe erasing power increases.
 11. A method for recording information inan optical recording medium for forming record marks in a recordinglayer containing a phase change material by modulating the power of alaser beam between a plurality of power levels including at leasterasing power, the method comprising a step of setting the level of theerasing power to a level higher than a level within an area where thereflection coefficient of the region between the neighboring recordmarks is not substantially changed even if the erasing power increasesand lower than a level within an area where the reflection coefficientof the region between the neighboring record marks moves away from thatof the record mark as the erasing power increases.
 12. A method forrecording information in an optical recording medium in accordance withclaim 11 wherein the optical recording medium is constituted as a“high-to-low type and the level of the erasing power is set to a levellower than a level within an area where the reflection coefficient ofthe region between the neighboring record marks increases as the erasingpower increases.
 13. A method for recording data in an optical recordingmedium for forming record marks in a recording layer containing a phasechange material by modulating the power of a laser beam between aplurality of power levels including at least erasing power wherein theerasing power is set to a first level when data are to be recorded at afirst linear velocity and the erasing power is set to a second leveldifferent from the first level when data are to be recorded at a secondlinear velocity higher than the first linear velocity.